Tài liệu Refreshing a DataSet Automatically Using Extended Properties pptx

A second DataTable with the auto-refreshing functionality is created that also contains the Categories table from the Northwind database.. The default view of the auto-refreshing table i

[ Team LiB ]

Recipe 9.14 Refreshing a DataSet Automatically Using Extended Properties

Problem

You need to automatically refresh a DataSet periodically

Solution

Use extended properties and a timer

The sample code contains two event handlers and one method:

Form.Load

Sets up the sample by creating a DataTable containing the Categories table from the Northwind database The default view of the table is bound to a data grid on the form A second DataTable with the auto-refreshing functionality is created that also contains the Categories table from the Northwind database The default view

of the auto-refreshing table is bound to a second data grid on the form An

extended property RefreshTime is added to the auto-refreshing table and set to 15 seconds—the value of the constant DATAREFRESH_SECONDS in the sample— into the future Finally, a thread timer is created with a TimerClassback delegate CheckRefreshDataSet, with a due time of one second, and a period of one second Update Button.Click

Uses a DataAdapter to update changes made to the first DataTable back to the data source

CheckRefreshDataSet( )

This method is called periodically by the thread timer The method checks whether the current time is later than the time in the RefreshTime extended property of the auto-refreshing table If it is, a DataAdapter is used to fill the table with the latest data from the data source and the RefreshTime extended property is once again set

to 15 seconds into the future

The C# code is shown in Example 9-17

Example 9-17 File: AutomaticRefreshDataSetForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Threading;

using System.Data;

using System.Data.SqlClient;

private const String CATEGORIES_TABLE = "Categories";

private const int DATAREFRESH_SECONDS = 15;

private const int DATASETCHECKREFRESHINTERVAL_MS = 1000;

private DataTable dt, dtRefresh;

private SqlDataAdapter da, daRefresh;

private System.Threading.Timer timer;

//

private void AutomaticRefreshDataSetForm_Load(object sender,

System.EventArgs e)

{

String sqlText =

"SELECT CategoryID, CategoryName, Description FROM Categories";

// Fill the categories table for editing

da = new SqlDataAdapter(sqlText,

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

SqlCommandBuilder cd = new SqlCommandBuilder(da);

dt = new DataTable(CATEGORIES_TABLE);

da.FillSchema(dt, SchemaType.Source);

da.Fill(dt);

// Bind the default view of the table to the grid

dataGrid.DataSource = dt.DefaultView;

// Fill the autorefresh categories table

daRefresh = new SqlDataAdapter(sqlText,

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

dtRefresh = new DataTable(CATEGORIES_TABLE);

daRefresh.FillSchema(dtRefresh, SchemaType.Source);

daRefresh.Fill(dtRefresh);

// Bind the default view of the table to the grid

refreshDataGrid.DataSource = dtRefresh.DefaultView;

// Set the refresh time for the data

dtRefresh.ExtendedProperties["RefreshTime"] =

DateTime.Now.AddSeconds(DATAREFRESH_SECONDS).ToString( );

// Start the timer

timer = new System.Threading.Timer(

new TimerCallback(CheckRefreshDatabase), null,

DATASETCHECKREFRESHINTERVAL_MS,

DATASETCHECKREFRESHINTERVAL_MS);

}

private void updateButton_Click(object sender, System.EventArgs e)

{

// Update the categories edited to the data source

da.Update(dt);

}

private void CheckRefreshDatabase(Object state)

{

DateTime now = DateTime.Now;

// Check if the specified number of seconds have elapsed

if (Convert.ToDateTime(dtRefresh.ExtendedProperties

["RefreshTime"].ToString( )) < now)

{

// Refresh the table

daRefresh.Fill(dtRefresh);

// Update the next refresh time

dtRefresh.ExtendedProperties["RefreshTime"] =

now.AddSeconds(DATAREFRESH_SECONDS).ToString( );

resultTextBox.Text = "Table refreshed (" + now.ToString("T") +

")" + Environment.NewLine + resultTextBox.Text;

}

}

Discussion

The ExtendedProperties property accesses a PropertyCollection of custom information for a DataSet, DataTable, DataColumn, DataRelation, or Constraint object The

PropertyCollection extends the Hashtable class to store information as a collection of key-and-value pairs The extended property data must be stored as strings; otherwise, it will not be persisted when the data is written as XML Add items to the collection using the Add( ) method, remove them with the Remove( ) method, and access them using the indexer in C# or the Item( ) property in VB.NET For more information about members

of the PropertyCollection class, see the MSDN Library

There are three timers in the Visual Studio NET and the NET Framework:

• The Windows-based timer System.Windows.Form.Timer (available on the

Windows Form tab of the Toolbox) is designed for a single-threaded environment where UI threads are used for processing This is the simplest timer to use but also the least accurate with an accuracy limited to 55ms

• The thread timer System.Threading.Timer is a simple, lightweight timer that uses callback methods to periodically run a task on a separate thread This timer can only be used programmatically This timer is more accurate than the Windows-based timer since the time interval between callbacks is specified in milliseconds

• The server-based timer System.Timers.Timer (available on the Components tab of

the Toolbox) is designed for use with worker threads in a multi-threaded

environment This timer uses server ticks generated out-of-process and is the most accurate of the three since the time interval between timer event firings is

specified in milliseconds

The thread timer uses a ThreadCallback delegate to specify the method to execute This delegate is specified when the Timer is constructed and cannot be changed The method executes in a thread pool supplied by the system rather than in the thread that created the timer

When the thread timer is created, the due time (the time to wait before first execution of the method) and the period (the amount of time to wait between subsequent executions)

are specified in the constructor A due time of 0 results in the callback being invoked immediately; a due time of Timeout.Infinite results in the callback method never being invoked A period value of 0 or Timeout.Infinite results in the callback invoked only once

as long as the due time is not infinite You can change the behavior of the timer at any time by using the Change( ) method When the timer is no longer needed, call the

Dispose( ) method to free its resources

[ Team LiB ]